1. Field of the Invention
The present invention relates to the fabrication of double-sided and multi-layer printed circuit boards, and more specifically, to a method of fabricating such structures using flexible film substrates which maintains the alignment between vias formed through the film and conductive pads on two sides of the film.
2. Description of the Prior Art
Double-sided and multi-layer printed circuit boards are now commonly used in the semiconductor industry to increase the density of integrated circuits. Such boards are typically formed from a planar substrate (or substrates) having printed circuitry on each side. The conductive layers on the surfaces of the substrate(s) are interconnected by conductive vias formed through the substrates. In a typical process flow, the vias (also referred to as "through holes" when they pass completely through a substrate) are drilled or punched through the substrate(s) followed by the electroless deposition of metal on the non-conductive surfaces of the holes. Conductive pads connected to either end of a via may be defined by exposure of a photoresist layer, followed by a plating step.
Flexible substrates formed from polyimide films are one type of substrate used to form double-sided and multi-layer printed circuit boards. The flexible circuitry so formed is an array of conductors bonded to a thin, flexible dielectric film (e.g., polyimide). Such structures have the property of being a three-dimensional circuit that can be shaped into multiplanar configurations, rigidized in specific areas, and molded to backer boards for specific applications. When used as an interconnect, the main advantages of flexible circuit based interconnects over traditional cabling are greater reliability, size and weight reduction, elimination of mechanical connectors, elimination of wiring errors, increased impedance control and signal quality, circuit simplification, greater operating temperature range, and higher circuit density. In many applications, lower cost is another advantage of using flexible circuits. In general, flexible circuits provide the benefits of reduced weight and increased circuit density over standard rigid printed circuit boards.
Although flexible circuitry has the advantages noted, certain problems are presented when flexible substrates are used during the formation of double-side circuits. In particular, the process flows conventionally used to form such structures have difficulty in maintaining correct alignment between vias (or through-holes) formed in the substrate and conductive pads formed on a surface of the substrate. This is because the substrate film typically shrinks or expands during fabrication of the circuitry, i.e., the film dimensions are not stable during the processes used to form the conductive vias and pads on the films. In the case of fine structure processes (i.e., processes used to form conductive pads having dimensions of less than 100 microns and line pitches less than 50 microns), even a small change in the substrate's dimensions can lead to mis-alignment between the vias and pads. As via and pad sizes are reduced to achieve higher density circuit features, the via/pad alignment requirement becomes more difficult to satisfy, especially for flexible substrate materials. In addition, conventional process flows are somewhat complicated, requiring two photoresist applications and exposures.
In conventional methods of forming vias and pads, the via formation step and trace/pad patterning step are separated by intermediate process steps, such as metallization. The intermediate steps can significantly change the dimensions of the substrate films and cause mismatch between vias and pads. For example, one conventional process, termed the "semi-additive" process involves: (1) preparation of a starting material (i.e., a dielectric film with or without copper cladding); (2) formation of a via or through hole; (3) cleaning of the via or through hole; (4) formation of a thin metallization layer (e.g., a seed layer, followed by sputtering or electroless plating); (5) coating of the substrate with photoresist and patterning; (6) plating of the defined pad regions; (7) stripping of the resist; and (8) performing a seed etch. In this process flow, the via formation and patterning steps are separated by a metallization step which can cause significant dimensional change in a flexible substrate.
A second conventional method of forming vias and pads is termed the "subtractive" process and involves: (1) preparation of a starting material (e.g., a dielectric film with or without copper cladding); (2) formation of a via or through hole; (3) formation of a thin metallization layer; (4) blanket electroplating of the substrate surface; (5) coating of the substrate surface with photoresist and patterning; and (6) etching of the patterned regions to form pads and lines. However, the above process has the problem that the via/pad registration can be affected by two metallization steps, which can impact the substrate dimensions and hence alignment between the pads and vias.
What is desired is a method of forming properly aligned, conductive vias between conductive features on either side of a flexible substrate which is suitable for fabrication of fine structure elements, and which overcomes the noted disadvantages of conventional processes.